High-pressure discharge lamps which provide light from vaporized mercury should provide effective light at a wavelength of 365 nm. This spectral line is also known as the I-line. Use of radiation at 365 nm permits high resolution of a mask work which is being irradiated. Some masks can also utilize the next higher effective spectral line at 436 nm wavelength. Details of the exposure methods and lamps used therein are described in the referenced U.S. Pat. No. 4,732,842, Kira, the disclosure of which is hereby incorporated by reference.
Lamps of this type have a problem in that they do not only radiate along the desired spectral radiation lines, for example, 436 nm and, especially, 365 nm, but, further, provide undesired radiation of wavelengths which are below 365 nm. Radiation at such lower wavelengths loads the overall system. These spectral components of lower wavelengths should, preferably, be suppressed. The customary way to suppress these lower wavelengths is the use of a filter in the radiation path--see, for example, the referenced Kira U.S. Pat. No. 4,732,842. Either an interference filter or a light color filter specific to the radiation to be suppressed can be used. When an interference filter is used, absorption gaps may result which is a disadvantage. These gaps occur due to adjacent maxima of the layer stack; when using a filter, loss of overall radiation intensity must be accepted.
As the exposure technology has been further developed and higher power lamps have been developed, for example, up to 5 kw or more, problems with respect to the undesired extra shortwave radiation assume increasing importance. The thermal loading, due to radiation absorption of the overall illumination system, forms a problem. Likewise, damage due to radiation itself, also known as solarization within the system and, especially, production of ozone by the lamp, form additional problems. Ozone production results in a chemical reaction, which may cause deposits on the optical components of the illumination system. Reactions due to ultraviolet radiation also occur, based in general on the ultraviolet content of the overall wavelength region below 365 nm.
It has previously been proposed to suppress ultraviolet (UV) spectral components in incandescent lamps by using doping materials for the lamp bulb. Usually, the lamp bulb is made of VYCOR (Reg. TM) or of hard glass. Incandescent lamps, however, provide continuous radiation of low intensity which rapidly decreases towards the lower wavelength. The referenced U.S. Pat. No. 3,253,174, Elmer et al., describes an incandescent lamp in which metal ions are used as doping materials of the glass. The doping materials are vanadium, cerium or molybdenum, employed in a quantity of between about 300 to 3000 ppm (by weight). The quantity described is with respect to a glass envelope of 1 mm thickness. This doping is used in a range of to about 300 nm and below. Surprisingly, it has been found that this doping is not suitable to solve the problem with mercury vapor high-pressure discharge lamps, since it already substantially dampens the desired radiation at 365 nm.
It has also been proposed to coat lamps with titanium dioxide, TiO.sub.2, see U.S. Pat. No. 4,985,275, Takemura et al. This coating, however, is effective primarily below about 240 nm and, due to the substantial distance from the desired radiation, can, at best, only be used as an additional additive.